1. Field of the Invention
The present invention relates to a spot welding system. The present invention also relates to a dressing determination method in the spot welding system.
2. Description of the Related Art
In a spot welding system, with respect to at least one pair of electrodes (hereinafter referred to as an electrode pair) provided in a welding gun, a shaping or forming process (typically referred to as a dressing) for restoring the surface of each electrode worn or deformed due to the repetition of a spot welding process to a proper shape is performed at required periods. In this connection, various dressing determination methods for determining the quality of the shaping process applied to the electrodes of the welding gun.
For example, Japanese Unexamined Patent Publication (Kokai) No. 2002-219581 (JP2002-219581A) discloses “method and device for detecting an abnormality in a dressing of an electrode tip provided in a welding gun” (Paragraph No. 0001). JP2002-219581A describes that “After a welding operation is performed by electrode tips for a predetermined number of times, a dressing operation is performed for the electrode tips, and thereafter, the wearing amount of the electrode tips is detected. Next, it is judged whether or not the detected wearing amount is within a reference wearing range defined based on a wearing amount of the electrode tips detected before the dressing operation is performed. Then, if the detected wearing amount is not within the reference wearing range, it is detected that the abnormality of the dressing occurs in the electrode tips.” (Paragraph No. 0008), “In response to a command from controller 14, electric welding gun 18 is driven so that electrode tips 20a, 20b are tentatively struck on each other before starting the welding operation (step S2).” (Paragraph No. 0023), “Encoder 54 disposed at a top of servo motor 28 detects a rotational angle of rotor 38 constituting the servo motor 28, and thereby detects a stroke of electrode tip 20a. Pulse signals from encoder 54 are converted to positional data 66 in encoder interface 64, and positional data 66 is sent to CPU 60 and tip position control section 68. In CPU 60, input positional data 66 is stored in tip position initial state storage section 80 as the initial positional states of electrode tips 20a, 20b (step S3).” (Paragraph No. 0025), “After the dressing operation is finished, similarly to the above, electrode tips 20a, 20b are tentatively struck on each other (step S7) and, based on the pulse signals from encoder 54, the wearing amount of electrode tips 20a, 20b are calculated (step S8).” (Paragraph No. 0028), and “It is judged whether or not a difference between wearing amount T1 of electrode tip 20a detected in a wearing amount detection (K1) after the dressing operation (D1) and wearing amount T2 of electrode tip 20a detected in a wearing amount detection (Ka) before the dressing operation (or an initial state detection (K0)) is within a predetermined reference wearing range.” (Paragraph No. 0031).
Japanese Patent No. 3289508 (JP3289508B) discloses “A welding tip management device attached to a welding gun” (Paragraph No. 0001). JP3289508B describes that “As illustrated in FIG. 1(B), there may be provided an initial tip diameter measurement means “f” for measuring a tip diameter of a welding tip after tip dressing, an initial tip diameter determination means “g” for comparing the tip diameter of the welding tip after tip dressing to upper and lower limit values and making a determination, a tip dressing re-instruction means “h” for instructing a repetitive tip dressing when it is judged that the tip diameter is not within a range between the upper and lower limit values, and a tip dresser abnormality determination means “i” for determining that there is abnormality of the tip dresser when the tip diameter is not within the above range even if the tip dressing is repeated for a predetermined number of times.” (Paragraph No. 0008), “As illustrated in FIG. 1(C), there may be provided a tip length measurement means “j” for measuring a length of the welding tip, and a tip exchange instruction means “k” for instructing to exchange the welding tip when the length of the welding tip becomes less than the lower limit value. Further, as illustrated by dotted lines in FIG. 1(B), there may be provided a tip length measurement means “j” for measuring a length of the welding tip, and an upper and lower limit value change means “m” for changing the upper limit value and the lower limit value for comparison in the initial tip diameter determination means “g” depending on the length of the welding tip.” (Paragraph No. 0009), “After the tip dressing, laser line sensor 9 measures the tip diameter (initial tip diameter) D of the welding tip in step 59. Then, in step 60, the measured initial tip diameter D is compared to upper limit value D0max and lower limit value D0min to determine whether the initial tip diameter D is within these values (D0min≦D≦D0max) or not.” (Paragraphs No. 0036), “After the tip dressing and the measurement, the process proceeds to step 65. In step 65, laser line sensor 9 measures the length L of the welding tip (the tip length). In step 66, the measured tip length L is compared to a predetermined lower limit value Lmin. If L<Lmin, the process proceeds to step 67 where an alarm is output to instruct the exchange of the welding tip” (Paragraph No. 0040).
In the conventional dressing determination method in a spot welding system, the quality of a shaping or forming process is determined by calculating the wearing amount of an electrode based on the pulse signals of the encoder of a welding gun before and after the shaping process for an electrode surface is performed, or by measuring the tip diameter and/or length of an electrode by using a laser line sensor after the shaping process for an electrode surface is performed. In these methods, if the pulse signals of the encoder do not accurately represent the position of the electrode due to, e.g., vibrations generating when the electrode is driven, or if the tip diameter or length of the electrode includes errors due to, e.g., deviation of the attachment position of the electrode with respect to the welding gun, the reliability of determination results is deteriorated.